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How To Model A Versioned Schema

Use this pattern for config files or messages that evolve over time and need an explicit version marker in the wire shape.

open CodecMapper

type SettingsV2 = {
    Version: int
    Mode: string
    Region: string option
}

let makeSettingsV2 version mode region =
    { Version = version; Mode = mode; Region = region }

let settingsV2Schema =
    Schema.record makeSettingsV2
    |> Schema.fieldWith "version" _.Version (
        Schema.int
        |> Schema.tryMap
            (fun value ->
                if value > 0 then Ok value
                else Error "version must be positive")
            id
    )
    |> Schema.field "mode" _.Mode
    |> Schema.field "region" _.Region
    |> Schema.build

Keep the version field explicit so schema evolution stays visible in one authored schema. If you also need omission policies or defaults, compose those helpers directly at the field boundary.